Electrosurgical generator power control circuit and method

ABSTRACT

A constant power control circuit for an electrosurgical generator and a method for maintaining the electrical power output of an electrosurgical generator at a generally constant value throughout a given tissue impedance range are disclosed. The constant power control circuit and the method recognize and use the unique and simple linear characteristics associated with certain electrosurgical generator designs to monitor and control the electrical power output without having to calculate or monitor the actual output power. The constant power control circuit includes a current sampling circuit, a linear conversion circuit, and a feedback correction circuit. The constant power control circuit may also include protection circuitry that prevents the electrosurgical generator from being over-driven during high and/or low impedance loading, and reduces the severity of exit sparking by providing a quick response to high impedance indications while nonetheless maintaining increased power levels throughout a preset, nominal impedance range. The constant power control circuit and method may be included as an integral part of the overall electrosurgical generator&#39;s circuitry, or may be embodied as a separate unit that connects to, and controls, an electrosurgical generator. The constant power control circuit and method may be embodied through a variety of analog and/or digital circuit components or arrangements, including software running on computational and memory circuitry.

RELATED APPLICATION INFORMATION

This application is a Continuation application of U.S. patentapplication Ser. No. 08/533,891 filed on Sep. 26, 1995, now U.S. Pat.No. 5,772,659.

FIELD OF THE INVENTION

A constant power control circuit for an electrosurgical generator and amethod for maintaining the electrical power output of an electrosurgicalgenerator at a generally constant level throughout a given tissueimpedance range.

BACKGROUND OF THE DISCLOSURE

An electrosurgical generator is used in surgical procedures to deliverelectrical energy to the tissue of a patient. An electrosurgicalgenerator often includes a radio frequency generator and its controls.When an electrode is connected to the generator, the electrode can beused for cutting or coagulating the tissue of a patient with highfrequency electrical energy. During normal operation, alternatingelectrical current from the generator flows between an active electrodeand a return electrode by passing through the tissue and bodily fluidsof a patient.

The electrical energy usually has its waveform shaped to enhance itsability to cut or coagulate tissue. Different waveforms correspond todifferent modes of operation of the generator, and each mode gives thesurgeon various operating advantage. Modes may include cut, coagulate, ablend thereof, desiccate, or spray. A surgeon can easily select andchange the different modes of operation as the surgical procedureprogresses.

In each mode of operation, it is important to regulate theelectrosurgical power delivered to the patient to achieve the desiredsurgical effect. Applying more electrosurgical power than necessaryresults in tissue destruction and prolongs healing. Applying less thanthe desired amount of electrosurgical power inhibts the surgicalprocedure. Thus, it is desirable to control the output energy from theelectrosurgical generator for the type of tissue being treated.

Different types of tissues will be encountered as the surgical procedureprogresses and each unique tissue requires more or less power as afunction of frequently changing tissue impedance. Even the same tissuewill present a different load impedance as the tissue is desiccated.

Two conventional types of power regulation are used in commercialelectrosurgical generators. The most common type controls the DC powersupply of the generator by limiting the amount of power provided fromthe AC mains to which the generator is connected. A feedback controlloop regulates output voltage by comparing a desired voltage with theoutput voltage supplied by the power supply. Another type of powerregulation in commercial electrosurgical generators controls the gain ofthe high-frequency or radio frequency amplifier. A feedback control loopcompares the output power supplied from the RF amplifier for adjustmentto a desired power level. Generators that have feedback control aretypically designed to hold a constant output voltage, and not to hold aconstant output power.

U.S. Pat. Nos. 3,964,487; 3,980,085; 4,188,927 and 4,092,986 havecircuitry to reduce the output current in accordance with increasingload impedance. In those patents, constant voltage output is maintainedand the current is decreased with increasing load impedance.

U.S. Pat. No. 4,126,137 controls the power amplifier of theelectrosurgical unit in accord with a non linear compensation circuitapplied to a feedback signal derived from a comparison of the powerlevel reference signal and the mathematical product of two signalsincluding sensed current and voltage in the unit.

U.S. Pat. No. 4,658,819 has an electrosurgical generator which has amicroprocessor controller based means for decreasing the output power asa function of changes in tissue impedance.

U.S. Pat. No. 4,727,874 includes an electrosurgical generator with ahigh frequency pulse width modulated feedback power control wherein eachcycle of the generator is regulated in power content by modulating thewidth of the driving energy pulses.

U.S. Pat. No. 3,601,126 has an electrosurgical generator having afeedback circuit that attempts to maintain the output current at aconstant amplitude over a wide range of tissue impedances.

None of the aforementioned U.S. Patents include a constant power controlcircuit that provides for a generally constant output power while alsoproviding a linear adjustment to account for the unique waveform crestfactors associated with different operational modes.

The preferred constant power control circuit and method provided hereinallows for output power control by way of a unique and simple linearconversion circuit coupled with protection circuitry that prevents theelectrosurgical generator from being over-driven during high and/or lowimpedance loading. The preferred constant power control circuit alsoreduces the severity of exit sparking by responding quickly to highimpedance indications while nonetheless maintaining substantiallyincreased power levels throughout a predetermined patient tissueimpedance range.

SUMMARY OF THE INVENTION

A constant power control circuit for use with an electrosurgicalgenerator. The constant power control circuit and method may be includedas an integral part of the overall electrosurgical generator'scircuitry, or may be designed as a separate unit that connects to, andcontrols, an electrosurgical generator. The constant power controlcircuit and method may be embodied through a variety of analog and/ordigital circuit components or arrangements, including software runningon computational and memory circuitry.

The constant power control circuit and method maintain the output powerof the electrosurgical current at a generally constant level over afinite patient tissue impedance range. The preferred patient tissueimpedance range is about 300 to 2500 ohms.

The constant power control circuit and method provide the capability tocontrol the output power of the electrosurgical generator without havingto actually monitor the amplitude of both the output current and outputvoltage. This allows for a simple constant power control circuit andmethod which operate to control the power output without having tocalculate the actual power output of the electrosurgical generator.

While the constant power control circuit may be used to controlelectrosurgical generators of varying designs, it is preferred that theelectrosurgical generator includes a power selection system wherein theuser may initialize, set, monitor, and/or control the operation of theelectrosurgical generator. It is also preferred that the power selectionsystem produces a control voltage signal that acts to control a highvoltage direct current supply which in turn acts to supply a highvoltage signal to an output switching radio frequency stage. Then outputswitching radio frequency stage creates an electrosurgical energybetween two output electrodes. The preferred electrosurgical generatorneed not be limited to these three functional elements, for example theelectrosurgical generator could also include additional safety,monitoring, signal modification/conditioning, and/or feedback circuitryor functional elements/processes. The actual electrosurgical generator'sdesign may include the use of digital components and signaling and/oranalogue components and signaling, or may be embodied, completely orpartially within a software process running on hardware components.

The constant power control circuit includes a current sampling circuit,a linear conversion circuit, and a feedback correction circuit. Thecurrent sampling circuit is coupled to one of the output electrodes, andfunctions so as to produce a sampled current signal that is proportionalto the average current flowing through the output electrode.

The linear conversion circuit which is connected to the current samplingcircuit internally generates one or more multiplier reference signalsand one or more offset reference signals, each of which is used tomodify the sampled current signal in accord with the crest factorassociated with the electrosurgical energy output by the electrosurgicalgenerator; the modified signal being a linear converted signal.

The feedback correction circuit which is electrically connected toreceive the linear converted signal from the linear conversion circuitand the control voltage signal from the power selection system functionsto produce a feedback control signal which it then supplies to the powerselection system, within the electrosurgical generator, so as to causethe power selection system to control the amount of electrosurgicalenergy created. The feedback correction circuit functions so as todetermine the difference in amplitude between the control voltage signaland the linear converted signal and to then add this difference to thecontrol voltage signal to produce a feedback control signal. Thefeedback correction circuit may also be connected to the primarytransformer winding within the output switching radio frequency stage,or its equivalent, thereby allowing the feedback correction circuit todetect high impedance loading between the output electrodes and toreduce the amplitude of the feedback control signal to protect thecircuitry and/or the patient from excessive current and/or voltagelevels. A high impedance load is generally considered to be above 2500ohms. The feedback correction circuit may also include circuitry orprocesses that substitute another signal for the feedback control signalwhen the impedance loading between the output electrodes is calculatedas being low. A low impedance load is generally considered to be below300 ohms. Both high and low impedance limits may be adjusted to matchthe instruments, processes, and/or procedures as necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents an electrosurgical generator interfaced to a constantpower control circuit having a current sampling circuit, linearconversion circuit and feedback correction circuit.

FIG. 2 is the preferred embodiment of the linear conversion circuitshown in FIG. 1.

FIG. 3 is the preferred embodiment of the feedback correction circuitshown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

For an electrosurgical generator 101 having a high voltage directcurrent (DC) supply 103 which is electrically connected to control anoutput switching radio frequency (RF) stage 105, a unique linearrelationship exists between the control voltage supplied to the highvoltage DC supply 103 and the root-mean-square (RMS) current generatedby the electrosurgical generator 101. This unique linear relationshipcan be used to design a constant power control circuit 107 thatfunctions as a feedback control loop to control the electrosurgicalgenerator 101. The following mathematical derivations define this uniquelinear relationship.

It can be shown that:

V_(control)=V_(dc)/K_(ps);

where,

V_(control)=a control voltage supplied to the high voltage DC supply,

V_(dc)=the output voltage signal of the high voltage DC supply, and

K_(ps)=a feedback ratio of the high voltage DC supply.

It can further be shown that:

V_(dc) ²×K_(a)=P_(out);

where,

P_(out)=the output power of the electrosurgical generator 101, and

K_(a)=a linear constant (which can be empirically derived).

Therefore, the output power of the electrosurgical generator 101 isdirectly proportional to the square of the output voltage signal of thehigh voltage DC supply.

Thus, by substitution:

(V_(control)×K_(ps))²×K_(a)=P_(out), or

V_(control) ²×K_(g)=P_(out);

where,

k_(g)=K_(ps) ^(2×K) _(a).

Therefore, the output power of the electrosurgical generator 101 isproportional to the square of the control voltage supplied to the highvoltage DC supply.

Examining the output of the generator we have:

P_(out)=V_(rms)×I_(rms);

where,

V_(rms)=output RMS voltage of the electrosurgical generator 101, and

I_(rms)=output RMS current of the electrosurgical generator 101.

Accordingly, at a given load impedance=R:

V_(rms)=I_(rms)×R,

and by substitution

P_(out)=I_(rms) ²×R.

By allowing R to equal a ‘matched’ load impedance we have

V_(control) ²×K_(g)=I_(rms) ²×R,

and therefore

V_(control) ²=I_(rms) ²×R/K_(g).

Consequently, for a given impedance K_(r)=R/K_(g) the equation can besimplified to:

V_(control) ²=I_(rms) ²×K_(r).

Therefore, the square of the control voltage supplied to the highvoltage DC supply 103 is directly proportional to the square of theoutput RMS current of the electrosurgical generator 101. It can also beshown by similar derivation that the square of the control voltagesupplied to the high voltage DC supply 103 is directly proportional tothe square of the output RMS voltage of the electrosurgical generator101.

Thus, the above derivation implies that if either the output RMS currentor voltage is sampled properly (I_(sample) & V_(sample) respectively)the control voltage supplied to the high voltage DC supply 103 may beused as a reference value in a feedback control loop to keep either theoutput RMS current or output RMS voltage constant. When the linearrelationship of I_(rms) to I_(sample) is ‘mapped’ into the linearrelationship of V_(control) to I_(rms) then a linear relationship can bederived between V_(control) and I_(samples). When the scaling is doneproperly for a given power setting, V_(control) will equal I_(sample) atthe ‘matched’ load impedance. Therefore, in a feedback circuit designedwith the above mapping a feedback loop which keeps I_(sample) equal toV_(control) will by definition keep I_(rms) constant.

In accord with the above presented mathematical derivation, we havedesigned a constant power control circuit 107 for the electrosurgicalgenerator 101, shown in FIG. 1, having a power selection system 109 thatproduces a control voltage signal to control a high voltage directcurrent supply 103 which supplies a high voltage signal to an outputswitching radio frequency stage 105 thereby creating an electrosurgicalenergy between two output electrodes 111. The preferred electrosurgicalgenerator 101 has a plurality of operational modes selectable within thepower selection system 109, and a primary transformer winding 113 withinthe output switching radio frequency stage 105, as shown in FIG. 1.

The constant power control circuit 107, shown in FIG. 1, includes acurrent sampling circuit 115, a linear conversion circuit 117 and afeedback correction circuit 119.

In the preferred embodiment, the current sampling circuit 115 isinductively coupled to one of the output electrodes 111, as shown inFIG. 1. Alternatively the current sampling circuit 115 could be activelycoupled, in circuit, with the output electrode.

The current sampling circuit 115 produces a sampled current signal thatis proportional in amplitude to the average current flowing from theelectrosurgical generator 101 through the one output electrode, animpedance load 121, and returning to the electrosurgical generator 101through another output electrode.

The preferred embodiment of the current sampling circuit 115 includes aninductive coil element, similar in design and function to that of asecondary winding of a current transformer. Additional circuit elementsfunction to transform the induced current into a proportional voltagesignal and include a voltage drop resistor, a calibrating variableresistor, and elements that rectify and average the sampled currentsignal.

The current sampling circuit 115 supplies the sampled current signal tothe linear conversion circuit 117. However, before the sampled currentsignal can be used as a feedback term, the mode crest factor for theselected electrosurgical generator 101 operational mode, needs to becompensated for. The linear conversion circuit 117, in FIGS. 1 and 2,compensates for the linear relationship between the sampled currentsignal and a ‘true’ sampled RMS value, which is of the formI_(rms)=m×I_(sample)+b, where I_(rms) is a signal which is directlyproportional to the RMS current, and m and b are given constants derivedfor a given crest factor. While electrosurgical generators 101 have awide variety of different output wave shapes with varying crest factors,it is preferred that the crest factor for a given mode be significantlyconstant over a finite patient tissue impedance range, such as between300 and 2500 ohms.

Accordingly, the linear conversion circuit 117 first multiples thesampled current signal by the gain, m, and then adds the offset to it,b. When the values of m and b are chosen properly the resulting linearconverted signal is directly proportional to the output RMS current ofthe electrosurgical generator 101. The preferred method for determiningthe proper values of m and b for a given operational mode andelectrosurgical generator 101 includes collecting empirical data on thecontrol voltage supplied to the high voltage DC supply 103 and theresulting output RMS current of the electrosurgical generator 101 andsolving the linear equation, for m and b, by substitution.

The linear conversion circuit 117, shown in FIGS. 1 and 2, iselectrically connected to the current sampling circuit 115. In thepreferred embodiment, the linear conversion circuit 117 is alsoelectrically connected to the power selection system 109 such that theoperational mode of the electrosurgical generator 101 can be determinedbased on this connection. The linear conversion circuit 117 generates alinear converted signal and supplies this signal to the feedbackcorrection circuit 119.

The preferred embodiment includes a linear multiplier generating means201 within the linear conversion circuit 117, see FIG. 2. The linearmultiplier generating means 201 generates a plurality of uniquemultiplier reference signals (i.e., a factor ‘m’). There is preferablyone, unique, multiplier reference signal for each operational mode. Thepreferred embodiment, of the linear multiplier generating means 201includes several resistive components connected to voltage sources,across which a predetermined voltage is maintained.

The preferred embodiment includes a linear offset generating means 203within the linear conversion circuit 117, see FIG. 2. The linear offsetgenerating means 203 generates a plurality of unique offset referencesignals (i.e., a factor ‘b’). There is preferably one, unique, offsetreference signal for each operational mode. The preferred embodiment ofthe linear offset generating means 203 includes several resistivecomponents connected to voltage sources, across which a predeterminedvoltage is maintained.

The preferred embodiment also includes a plurality of multipliers 205,within the linear conversion circuit 117, see FIG. 2. There ispreferably one, corresponding, multiplier 205 for each operational mode.Each multiplier 205 is electrically connected to receive the sampledcurrent signal and one unique multiplier reference signal from thelinear multiplier generating means 201. Each multiplier 205 multipliesthe sampled current signal and the unique multiplier reference signalassociated with one operational mode to produce a unique multipliedsignal for that operational mode. The preferred embodiment of themultiplier 205 includes a plurality of operational amplifiers.

The preferred embodiment includes a plurality of summers 207, within thelinear conversion circuit 117, see FIG. 2. There is preferably one,corresponding, summer 207 for each operational mode. Each summer 207 iselectrically connected to receive a unique multiplied signal and oneunique offset reference signal from the linear offset generating means203. Each summer 207 sums the offset reference signal associated withone operational mode and the unique multiplied signal associated withthat operational mode to produce a unique linear converted signal forthat operational mode. The preferred embodiment of the summer 207includes configuring the plurality of operational amplifiers used asmultipliers 205 to also function as summers 207.

The preferred embodiment includes a mode monitor 209, within the linearconversion circuit 117, see FIG. 2. The mode monitor 209 is electricallyconnected to the power selection system 109, for identifying theoperational mode of the electrosurgical generator 101 and producing anidentified operational mode signal therefrom.

Closely associated with the mode monitor 209, is a signal selector 211that is also within the linear conversion circuit 117, see FIG. 2. Thesignal selector 211 is electrically connected to receive the identifiedoperational mode signal and the unique linear converted signal from eachof the summers 207. The signal selector 211 selects the unique linearconverted signal associated with the identified operational mode, andcauses that linear converted signal to be supplied to the feedbackcorrection circuit 119. In the preferred embodiment the mode monitor 209and signal selector 211 are embodied within a circuit including adigital processing component that activates and/or deactivates aplurality of electronic switching elements.

The feedback correction circuit 119, shown in FIGS. 1 and 3, iselectrically connected to receive the linear converted signal from thelinear conversion circuit 117, the control voltage signal from the powerselection system 109, and the voltage signal across the primarytransformer winding 113. The feedback correction circuit 119 produces afeedback control signal and supplies the feedback control signal to thepower selection system 109 so as to control the amount ofelectrosurgical energy created by the electrosurgical generator 101.

The feedback correction circuit 119 includes a subtractor 301, see FIG.3. The subtractor 301 is electrically connected to receive the linearconverted signal from the linear conversion circuit 117 and the controlvoltage signal which is generated by the power selection system 109 andsupplied to the high voltage DC supply, see FIGS. 1 and 3. Thesubtractor 301 determines the difference in amplitude between thecontrol voltage signal and the linear converted signal, and produces adelta signal proportional to the difference. The preferred embodiment ofthe subtractor 301 includes an operational amplifier component.

Also included in the feedback correction circuit 119 is an adder 303,see FIG. 3. The adder 303 is electrically connected to receive the deltasignal and the control voltage signal. The adder 303 adds the deltasignal to the control voltage signal to produce the feedback controlsignal. The preferred embodiment includes an operational amplifiercomponent.

Since holding the output RMS current constant for all impedances wouldbe a physical impossibility based on the design limitations of the highvoltage DC supply 103 and the output switching RF stage 105, it ispreferred that the feedback control signal to the high voltage DC supply103 be limited as a function of the impedance load 121 between theoutput electrodes 111.

In the preferred embodiment, the feedback correction circuit 119includes a maximum control voltage reference generator 305 forgenerating a maximum control voltage reference signal, see FIG. 3. Thepreferred embodiment uses an operational amplifier component connectedto the control voltage signal to establish a maximum control voltagereference signal based thereon.

The maximum control voltage reference signal is supplied to a switcher307 within the preferred feedback correction circuit 119, see FIG. 3.The switcher 307 is also electrically connected to receive the feedbackcontrol signal from the adder 303. The switcher 307 substitutes themaximum control voltage reference signal for the feedback control signalwhen the feedback control signal is greater in amplitude than themaximum control voltage reference signal, thereby limiting theelectrosurgical generator's 101 output current through the outputelectrodes 111 when the impedance load 121 is at a low impedance level.The preferred embodiment of the switcher 307 includes an AND circuitcreated with diodes that passes the lower of the two signals as thefeedback control signal.

When the impedance load 121 between the output electrodes 111 is high,the preferred constant power control circuit 107 should limit the outputvoltage of the electrosurgical generator 101 so as protect theelectrosurgical generator 101, and reduce leakage currents and exitsparking.

In the preferred embodiment, the feedback correction circuit 119 shownin FIG. 3, includes a high impedance reference generator 309 forgenerating a high impedance reference signal. The high impedancereference generator 309 is electrically connected to receive the controlvoltage signal. The preferred high impedance reference generator 309establishes the high impedance reference signal by linearly convertingthe control voltage signal with an operational amplifier.

In the preferred embodiment a connector 311 is used for electricallyconnecting a comparator 313, within the feedback correction circuit 119,to the primary transformer winding 113, see FIGS. 1 and 3. The connector311 provides the comparator 313 with the voltage across the primarytransformer winding 113. The comparator 313 is also electricallyconnected to receive the high impedance reference signal. The comparator313 compares the amplitude of the high impedance reference signal to thevoltage across the primary transformer winding 113 and produces a highimpedance detection signal that indicates the results of thiscomparison. In the preferred embodiment the comparator 313 includes anoperational amplifier component.

The high impedance detection signal is received by a reducer 315, shownin FIG. 3 of the preferred embodiment, which is electrically connectedto the comparator 313 and to the switcher 307. The reducer 315 reduces,to an internally generated preset reduced voltage level signal, theamplitude of the feedback control signal from the switcher 307 when thevoltage across the primary transformer winding 113 is greater than thehigh impedance reference signal as indicated by the high impedancedetection signal. In the preferred embodiment, the reducer 315 includesa logic driven switched circuit and an adjustable resistor providing areduced voltage level signal. The reducer 315 supplies the resultingfeedback control signal to the power selection system 109.

Associated with the constant power control circuit 107 is a method formaintaining a generally constant output power from an electrosurgicalgenerator 101 having a power selection system 109 that produces acontrol voltage signal to control a high voltage direct current supply103 which supplies a high voltage signal to an output switching radiofrequency stage 105 thereby creating an electrosurgical energy betweentwo output electrodes 111.

The method includes the steps of inductively coupling to one outputelectrode, sensing the current flowing through the output electrode 111and producing a sampled current signal proportional to the averagecurrent flowing through the output electrode. The method then continueswith the steps of generating a multiplier reference signal, generatingan offset reference signal, multiplying the sampled current signal andthe multiplier reference signal, and then summing the offset referencesignal to the product to producing a linear converted signal.

The method continues with the steps of connecting to the control voltagesignal from the power selection system 109, determining the differencein amplitude between the control voltage signal and the linear convertedsignal, adding the difference determined by the subtraction means to thecontrol voltage signal to produce a feedback control signal, and thensupplying the feedback control signal to the power selection system 109to control the amount of electrosurgical energy created.

To protect the electrosurgical generator 101 and the patient when theimpedance load 121 is high, the method can include the steps ofgenerating a high impedance reference signal, connecting to the primarytransformer winding 113, comparing the amplitude of the high impedancereference signal to the voltage across the primary transformer winding113, and reducing the amplitude of the feedback control signal when thevoltage across the primary transformer winding 113 is greater than thehigh impedance reference signal.

To protect the electrosurgical generator 101 and patient when theimpedance load 121 is low, the method can include the steps ofgenerating a maximum control voltage reference signal and substitutingthe maximum control voltage reference signal for the feedback controlsignal when the feedback control signal is greater in amplitude than themaximum control voltage reference signal.

For electrosurgical generators 101 having a plurality of operationalmodes, the method can be modified to include the steps of generating aplurality of unique linear multiplier reference signals, one for eachoperational mode, and generating a plurality of unique linear offsetreference signals, one for each operational mode. The method would theninclude the steps of multiplying the sampled current signal, separatelyand concurrently, with each of the unique multiplier reference signalsto produce a plurality of unique multiplied signals, one for eachoperational mode, and then summing each of the unique multiplied signalswith the offset reference signal associated with the same operationalmode to produce a plurality of unique linear converted signals, one foreach operational mode. The method would continue with the steps ofconnecting to the power selection system 109 to identify the operationalmode selected, selecting the unique linear converted signal that matchesthe identified operational mode, and then causing that linear convertedsignal to be supplied to the feedback correction circuit 119.

What is claimed is:
 1. A control circuit for providing a constant poweroutput in an electrosurgical generator, the generator having a powerselection system which supplies a high voltage output to createelectrosurgical energy at a pair of output electrodes, the controlcircuit comprising: a current sampling circuit inductively coupled toone of the output electrodes; a linear conversion circuit coupled to thecurrent sampling circuit and the power selection system of theelectrosurgical generator; and a feedback correction circuit coupled tothe liner conversion circuit, the power selection system and aradiofrequency stage, the feedback correction circuit being adapted toreceive a control voltage signal from the power selection system and alinear converted signal from the linear conversion circuit to produce afeedback control signal, the feedback control signal adapted to besupplied to the power selection system to control electrosurgical energycreated at the pair of output electrodes.
 2. A control circuit accordingto claim 1, wherein the current sampling circuit produces a signalproportional to an average current flowing through the one outputelectrode.
 3. A control circuit according to claim 1, wherein the radiofrequency stage includes a primary transformer winding, the feedbackcorrection circuit being coupled to the primary transformer winding toadjust the amplitude of the feedback control signal.
 4. A method formaintaining a generally constant output power from an electrosurgicalgenerator having a power selection system which supplies a high voltagesignal to create electrosurgical energy between two output electrodes,the method including the steps of: inductively coupling to one outputelectrode; sensing the current flowing through the output electrode;producing a sampled current signal proportional to the average currentflowing through the output electrode; producing a linear convertedsignal; providing a control voltage signal from the power selectionsystem; producing a feedback control signal from the control voltagesignal and the linear converted signal; and supplying the feedbackcontrol signal to the power selection system to control the amount ofelectrosurgical energy created.
 5. A constant power control circuit foran electrosurgical generator having a power selection system and capableof functioning in one or more operational modes comprising: a currentsampling circuit coupled to an output electrode for generating a currentsignal; a linear conversion circuit electrically connected to thecurrent sampling circuit for receiving and adjusting the current signalto compensate for the operational mode of the electrosurgical generator,the linear conversion circuit generating a linear converted signal; anda feedback correction circuit electrically connected to the linearconversion circuit for receiving the linear converted signal, thefeedback correction circuit comparing the linear converted signal to acontrol voltage signal and generating a feedback control signal tocontrol the amount of electrosurgical energy created.
 6. The controlcircuit according to claim 5, wherein the feedback correction circuitincludes a reducer to reduce the amplitude of the feedback controlsignal if a high impedance load is detected in the circuit.
 7. Thecontrol circuit according to claim 6, wherein the reducer iselectrically connected to a comparator which receives a high impedancereference signal from a high impedance reference generator and comparesit to an output voltage to generate an impedance detection signal. 8.The control circuit according to claim 7, wherein the reducer receivesthe impedance detection signal and reduces the amplitude of the feedbackcontrol signal to a preset reduced voltage level signal if the outputvoltage is greater than the impedance detection signal to therebyprotect the patient from excessive voltage levels if the impedance ishigh.
 9. The control circuit according to claim 5, wherein the feedbackcorrection circuit includes a generator for generating a maximum controlvoltage reference signal, wherein the maximum control voltage referencesignal is substituted for the feedback control signal if the feedbackcontrol signal is greater in amplitude than the maximum control voltagereference signal, thereby limiting output current of the electrosurgicalgenerator if the impedance is low.
 10. The control circuit according toclaim 5, wherein the feedback correction circuit compares the linearconverted signal to the control voltage signal by determining thedifference in amplitude between the control voltage signal and thelinear converted signal to produce a delta signal proportional to thedifference, and subsequently adds the delta signal to the controlvoltage signal to produce the feedback control signal.
 11. The controlcircuit according to claim 5, wherein the feedback correction circuitincludes a generator for generating a high impedance reference signal bylinearly converting the control voltage signal.
 12. The control circuitaccording to claim 5, wherein the feedback control signal is supplied tothe power selection system to control the amount of electrosurgicalenergy created.
 13. The control circuit according to claim 5, whereinthe current signal generated by the current sampling circuit is producedin proportion to the amplitude of average current flowing through theoutput electrode.
 14. The control circuit according to claim 5, whereinthe linear converted signal is produced by multiplying the currentsignal by a first constant and adding a second constant to themultiplied signal.
 15. The control circuit according to claim 14,wherein the first and second constants are predetermined valuesdetermined by the operational mode of the electrosurgical generator,thereby compensating for the various mode crest factors.
 16. The controlcircuit according to claim 15, wherein the linear converted signal isdirectly proportional to the output RMS current of the electrosurgicalgenerator.
 17. The control circuit according to claim 15, wherein thefeedback correction circuit includes a mode monitor electricallyconnected to the electrosurgical generator for producing an operationalmode signal to identify the operational mode of the electrosurgicalgenerator.
 18. The control circuit according to claim 5, wherein theelectrosurgical generator includes a radio frequency output stage. 19.The control circuit according to claim 5, wherein the linear conversioncircuit is electrically connected to the power selection system so thatthe operational mode of the electrosurgical generator can be determined.20. A power control circuit for an electrosurgical generator comprisingmeans for controlling the output voltage in response to the circuitimpedance load by adjusting a feedback control signal, the feedbackcontrol signal controlling the output of the electrosurgical generator,the controlling means including a correction circuit having a reducerand a comparator for comparing the amplitude of a high impedancereference signal to the output voltage, wherein the reducer reduces theamplitude of the feedback control signal to a preset reduced voltagelevel signal if the output voltage is greater than the amplitude of thehigh impedance reference signal.
 21. The control circuit according toclaim 20, wherein a high impedance detection signal is generated by thecomparator indicative of the comparison.
 22. The control circuitaccording to claim 21, wherein the high impedance reference signal isgenerated by a high impedance reference generator, the high impedancereference generator being electrically connected to receive a controlvoltage signal from the electrosurgical generator and linearlyconverting the control voltage signal.
 23. A power control circuit foran electrosurgical generator comprising means for controlling the outputcurrent in response to the impedance load, the controlling meansincluding a correction circuit having a switcher, the switcher comparingan amplitude of a feedback control signal which controls the output ofthe electrosurgical generator to an amplitude of a maximum controlvoltage reference signal and substituting the maximum control voltagereference signal if the amplitude of the feedback control signal exceedsthe amplitude of the maximum control voltage reference signal to therebylimit the output current when an impedance load is at a low level. 24.The control circuit according to claim 23, wherein the maximum controlreference signal is generated by a maximum control voltage referencegenerator electrically connected to the switcher.
 25. A control circuitaccording to claim 24, wherein the correction circuit further comprisesa reducer to reduce the feedback control signal to a preset value inresponse to a high impedance reference signal.
 26. A control circuitaccording to claim 25, wherein the feedback control signal will bereduced to a preset reduced level voltage signal by the reducer if theoutput voltage is greater than an amplitude of the high impedancereference signal.
 27. A control circuit according to claim 26, wherein ahigh impedance detection signal is generated by a comparatorelectrically connected to the reducer for comparing the amplitude of thehigh impedance reference signal to a voltage across a primarytransformer winding within an output switching radio frequency stage ofthe electrosurgical generator.